Method And Installation For The Emergency Back-Up Supply Of A Gas Under Pressure

ABSTRACT

In a method for the emergency back-up supply of a gas under pressure by vaporization of a pressurized liquid, this gas normally being supplied by vaporization of liquid ( 33 ) in a first exchanger ( 1 ) of a pumped air separation unit, during the step of operating a second exchanger ( 2 ) in order to produce the make-up gas, pressurized liquid ( 33 ) and high-pressure air ( 8 ) continue to be sent to the first exchanger.

This invention relates to methods and installations for the emergency back-up supply of a gas under pressure, by vaporization of cryogenic liquids, in particular those used for supplying customers with gaseous products (nitrogen, oxygen, argon) when the industrial installations (such as air separation units) can ensure only partial supply of the product, or even no supply at all (f or example in the event of trip-out, load reduction for an electricity tariff constraint, etc.). The invention also applies to the storage of other cryogenic liquids, such as hydrogen, helium and carbon monoxide.

An emergency back-up vaporizer is illustrated in EP-A-0 452 177 in which liquid nitrogen coming from a storage tank is vaporized in an exchanger by heat exchange with the ambient air.

EP-A-0 628 778 discloses a cryogenic liquid storage tank in which the liquid is pumped and then vaporized in a vaporizer before being sent to the customer.

“Large Oxygen Plant Economics and Reliability” by W. J. Scharle, Bulletin Y-143, National Fertilizer Division Center, Tennessee Valley Authority, Muscle Shoals, Ala. and “Oxygen Facilities for Synthetic Fuel Projects” by W. J. Scharle and K. Wilson, Journal of Engineering for Industry, November 1981, Vol. 103, pp. 409-417 describe an emergency back-up oxygen production system composed of:

a storage tank containing a quantity of product in liquid form;

several pumps (here, two pumps for reliability reasons) that withdraw the liquid contained in the storage tank in order to compress it to the pressure normally delivered to customers (pressure in the line); and

an exchanger, the function of which is to vaporize the liquid under pressure.

On leaving this equipment, the gas is in general close to the ambient temperature and is sent to the customer. Depending on the energy sources available on the site and their costs, this exchanger may use as heat source to vaporize the liquid under pressure, for example air, steam, hot water or combustion flue gas.

One of the main features of these emergency back-up installations is their start-up time. This is particularly important as it determines the quality and the continuity of the gas supply to customers. An excessively long start-up time after tripping of the production unit may cause too great a pressure drop in the line and may generate malfunctions in customer processes and installation shut-down.

In the case of the oxygen production systems described in the above articles, a gaseous oxygen buffer tank is provided in order to supply the pressurized product during the time needed to bring the pump into operation if the pump has to be operated cold (about 15 to 20 minutes according to the abovementioned articles by W. J. Scharle).

Conventionally, if the vaporization pump is permanently maintained at cryogenic temperature and if the distance between the pump and the vaporization hairpin is very short, the time needed for the emergency back-up system to reach 100% of its capacity in a stable manner is around 2 minutes, made up by 1 minute for the pump to start up and 1 minute for the vaporization exchanger to come up to speed. In certain cases, this time of 2 minutes is still too long as regards permitted pressure fluctuation constraints in the line—in this case, as described above, one solution consists in installing, downstream of the exchanger, gas buffer tanks (for example at 200 bar) designed to supply the production for 1 to 3 minutes, the time that the system made up of the pump and the vaporizer requires to reach its normal operating speed. The drawback of this solution is its high price (large volume, high pressure, pump for filling the buffer tanks, etc.).

FR-A-2 825 136 describes a method for the emergency back-up supply of a gas under pressure by vaporization of a pressurized liquid in which the liquid to be pressurized is stored in a storage tank, liquid is withdrawn from the storage tank and pressurized, and at least some of the pressurized liquid is vaporized in a vaporizer in order to produce the emergency back-up gas under pressure and, if the flow of gas under pressure is not required, liquid is withdrawn from the storage tank and pressurized, some of the pressurized liquid is vaporized in a vaporizer and the rest of the pressurized liquid is returned to the storage tank after depressurization.

Partial oxidation reactors require a supply of oxygen at high pressure (70 bar and higher) with the pressure stabilized to ±1% of the nominal value. Air separation units supplying the oxygen must therefore comply with this constraint whatever their operating mode and in particular in the event of the air separation unit being shut down.

During the time to bring the emergency back-up vaporization unit into service, the pressure in the customer's network will drop, following a curve whose slope depends on the volume of water in the network and on the flow consumed. Therefore the low pressure limit (−1%) may be rapidly reached (in less than 5 seconds) if the length of the customer's network is less than one kilometre.

It is therefore necessary to have an oxygen supply system that provides the necessary flow to the customer during start-up of the vaporization hairpin, the pumps already being in operation.

It is an object of the invention to provide a method for the emergency back-up supply of a gas under pressure, by vaporization of a pressurized liquid in which:

i) during a first step:

-   -   a) a first stream of air is compressed by means of a compressor         and purified in a purification unit;     -   b) at least one portion of the air is compressed to a high         pressure, which allows vaporization of the pressurized liquid;     -   c) at least one portion of the air at the high pressure is sent         to a first exchanger;     -   d) at least one portion of the air cooled in the exchanger is         sent to a system of distillation columns;     -   e) at least one liquid is withdrawn from one of the columns of         the system, the liquid is pressurized and sent to the exchanger,         where it vaporizes to form a gas under pressure;

ii) during a second step:

-   -   a stream of the said liquid is sent to a second exchanger where         it vaporizes to form a make-up stream of the gas under pressure,         characterized in that, during the second step, at least         initially, air at the high pressure and pressurized liquid         coming from one column of the system are sent to the first         exchanger and then the stream of high-pressure air sent to the         first exchanger is reduced, possibly to zero, and the stream of         pressurized liquid sent to the first exchanger is reduced,         possibly to zero.

According to other optional aspects:

during at least part of the second step, air is sent from a gaseous air storage tank to the first exchanger and then to the system of columns and, optionally during the first step, supercharged air is sent to an air storage tank where it is stored in gaseous form;

all the air is compressed to the high pressure and then purified in the purification unit, the purification unit constituting the air storage tank;

all the liquid to be pressurized is stored in a second storage tank;

during the first step, a stream of the said liquid, smaller than the stream sent during the second step, is sent to the second exchanger where it vaporizes to form gas under pressure;

at least one of the liquids to be pressurized is rich in oxygen, argon, nitrogen, hydrogen, helium, methane or carbon monoxide;

the liquid is pressurized by means of at least one pump;

during the second step, air is sent directly from the first storage tank to the first exchanger; and

during a third step, pressurized liquid is sent only to the second exchanger and air is no longer sent to the first exchanger.

It is another object of the invention to provide an installation for the emergency back-up supply of a gas under pressure, by vaporization of a pressurized liquid, comprising:

-   -   i) a first storage tank;     -   ii) a pump;     -   iii) a first exchanger;     -   iv) a second exchanger;     -   v) a compressor;     -   vi) a purification unit;     -   vii) a supercharger;     -   viii) means for sending air to the compressor, means for sending         compressed air to the purification unit and means for sending at         least one portion of the purified air to the supercharger;     -   ix) means for sending air to a column of a system of columns;     -   x) means for withdrawing at least one liquid from the system of         columns, optionally after having stored it in a storage tank;     -   xi) means for sending the liquid to the pump in order to         pressurize it;     -   xii) means for sending the pressurized liquid to the first         exchanger;     -   xiii) means for withdrawing the vaporized liquid from the first         exchanger;     -   xiv) means for sending the pressurized liquid to the second         exchanger; and     -   xv) a tank for storing gaseous air under pressure, the tank         being connected to the outlet of the air supercharger,         characterized in that it includes means for sending the air from         the supercharger to the storage tank.

The invention will be described in greater detail with reference to FIGS. 1 to 5. FIGS. 1 and 5 show an air separation unit according to the invention and FIGS. 2 to 4 show the air and oxygen streams during various steps of the method according to the invention.

The Figures denoted A show the streams of high-pressure air and pressurized oxygen in the first exchanger, while the figures denoted B show the pressurized oxygen stream in the second exchanger.

FIG. 1 shows an air separation unit with a double column 15, 17, the medium-pressure and low-pressure columns being thermally coupled by a condenser 21.

An air stream is compressed to the medium pressure by a compressor 3 and then purified in the purification unit 5. The purified stream is divided into two. One portion is sent to a supercharger 7 where it is supercharged to a high pressure of between 20 and 100 bar. The rest of the air 13 is sent to the first exchanger 1, where it cools before being sent to the medium-pressure column 15.

The reflux streams are not shown in order to simplify the figure.

A liquid oxygen stream 27 is withdrawn from the bottom of the low-pressure column 17 and sent to the storage tank 19.

A gaseous nitrogen stream 23 is withdrawn from the top of the low-pressure column 17 and used to regenerate the purification unit 5.

During a first step, which constitutes the ordinary operation of the air separation unit, a small portion of the supercharged air is sent to the storage tank 9 so as to fill it via the line 25.

Otherwise, the remaining supercharged air is sent to the first exchanger 1 where it condenses before being sent to the double column.

Air withdrawn from the storage tank 19 is pressurized by the pump 39 and sent to the first exchanger 1 via the line 33, where it vaporizes to form gaseous oxygen under pressure.

Optionally, a small stream of oxygen may be permanently sent during the first step to the second exchanger 2, where it vaporizes by heat exchange with a heat-transfer fluid independent of the air separation unit, such as steam or the ambient air.

When it is desired to shut down the air separation unit, the operation switches to the second step and the stream of liquid oxygen sent to the first exchanger 1 is progressively reduced, while the stream sent to the second exchanger 2 is increased progressively so as to ensure a gentle transition towards the second exchanger 2. To vaporize the liquid oxygen in the first exchanger 1, the liquid air valve 41 on the line 11 remains open, controlled according to the oxygen flow rate and the air pressure. This air pressure drops gently from the nominal pressure at the outlet of the supercharger 7 down to a value close to that of the medium-pressure column 15.

FIG. 2A shows the oxygen and supercharged air streams sent to the first exchanger during the second step, in the case in which no oxygen stream is sent to the second exchanger during the first step.

Air in the storage tank 9 is sent to the first exchanger 1 immediately after the compressor 3 and/or the supercharger 7 are/is stopped. The supercharged air stream is instantly increased, to compensate for the other heat-supplying streams absent (as there is no longer any medium-pressure air 13), by opening the liquid air valve 41. Thus, firstly the supercharged air stream increases to above the nominal value during the first step (100%) and then progressively reduces as the stream of vaporized oxygen is reduced, and likewise the pressure in the supercharged air passage of the exchanger decreases, this pressure being controlled by the make-up from the gas storage tank, likewise the pressure in the air storage tank decreases. The supercharged air stream is linearly reduced, by closing the liquid air valve 41, according to the stream of high-pressure gaseous oxygen which is itself also linearly reduced when the oxygen stream sent to the second exchanger is itself linearly increased. One minute passes between the triggering of the air stream coming from the storage tank and the reduction to zero of the supercharged air sent to the first exchanger. Some of the heat needed to vaporize the liquid oxygen comes from the thermal inertia of the main exchanger.

FIG. 3A shows the oxygen and supercharged air streams sent to the first exchanger during the second step in the case in which an oxygen stream is sent to the second exchanger during the first step.

FIG. 3B shows that, during the second step, the oxygen stream vaporized in the second exchanger 2 starts from a value of Y %, which is the value of the stream sent during the first step, and rises to 100% with a transition time of only 15 seconds.

FIG. 4A shows that no air or oxygen stream is sent to the first exchanger 1 during the third step. FIG. 4B shows that all the pressurized oxygen is sent to the second exchanger during the third step.

In the examples of FIGS. 1 to 4, the first air storage tank 9 is at a higher pressure than the output pressure of the supercharger 7—it has therefore been increased by an auxiliary system, for example a small compressor. With this higher-pressure, for example 150 bar, system, it is possible to maintain the pressure in the supercharged passage at the nominal value or to allow it to drop.

In the installation shown in FIG. 5, which is less expensive than that of FIG. 1, there is no system for increasing the pressure of the first storage tank 9. The first storage tank 9 is connected to the outlet of the supercharger 7. When the compressor 3 and/or the supercharger 7 are/is stopped, the method of supplying make-up gas is the same as that described previously—the supercharged air stream is immediately increased by opening the HP liquid air valve 41, and then it is reduced as the oxygen stream reduces and the air pressure reduces.

Of course, it is possible for the installation to be of the type described in EP-504 029 with all the air intended for distillation being compressed to a single pressure, purified, sent to the first exchanger, where it exchanges heat with the oxygen, and is then sent partly to the distillation unit, the rest of the air being expanded in a Claude turbine. 

1-12. (canceled)
 13. A method for the emergency back-up supply of a gas under pressure, by vaporization of a pressurized liquid in which: i) during a first step: a) a first stream of air is compressed by means of a compressor (3) and purified in a purification unit (5); b) at least one portion of the air is compressed to a high pressure, which allows vaporization of the pressurized liquid; c) at least one portion of the air at the high pressure is sent to a first exchanger (1); d) at least one portion of the air cooled in the first exchanger is sent to a system of distillation columns (15, 17); and e) at least one liquid (27) is withdrawn from one of the columns of the system, the liquid is pressurized and sent to the first exchanger, where it vaporizes to form a gas under pressure; ii) during a second step a stream of the said liquid is sent to a second exchanger (2) where it vaporizes to form a make-up stream of the gas under pressure, characterized in that, during the second step, at least initially, air at the high pressure and pressurized liquid coming from one column of the system are sent to the first exchanger and then the stream of high-pressure air sent to the first exchanger is reduced, possibly to zero, and the stream of pressurized liquid sent to the first exchanger is reduced, possibly to zero.
 14. The method of claim 13, in which, during at least part of the second step, air is sent from a gaseous air storage tank (5, 9) to the first exchanger and then to the system of columns and, optionally during the first step, supercharged air is sent to the air storage tank where it is stored in gaseous form.
 15. The method of claim 14, in which all the air is compressed to the high pressure and then purified in the purification unit (5), the purification unit constituting the air storage tank.
 16. The method of claim 15, in which, as high-pressure air passages, only passages of the first exchanger (1) dedicated to high-pressure air are used, these passages being closest to the passages for vaporizing the said liquid, this being achieved by separation of the liquid air output boxes at the cold end of the main exchanger and by the addition of a liquid air valve per additional liquid air output box, which makes it possible to expand this liquid into the columns.
 17. The method of claim 13, in which all the liquid to be pressurized is stored in a second storage tank (19).
 18. The method of claim 13, in which, during the first step, a stream of the said liquid, smaller than the stream sent during the second step, is sent to the second exchanger (2) where it vaporizes to form gas under pressure.
 19. The method of claim 13, in which at least one of the liquids (27) to be pressurized is rich in oxygen, argon, nitrogen, hydrogen, helium, methane or carbon monoxide.
 20. The method of claim 13, in which the liquid is pressurized by means of at least one pump (39).
 21. The method of claim 13, in which, during the second step, air is sent directly from the first storage tank (5, 9) to the first exchanger (1).
 22. The method of claim 13, in which, during a third step, pressurized liquid is sent only to the second exchanger (2) and air is no longer sent to the first exchanger (1).
 23. The method of claim 13, in which, during the second step, at least some of the heat needed to vaporize the liquid comes from the thermal inertia of the first exchanger (1).
 24. An installation for the emergency back-up supply of a gas under pressure, by vaporization of a pressurized liquid, comprising: i) a first storage tank (5, 9); ii) a pump (39); iii) a first exchanger (1); iv) a second exchanger (2); v) a compressor (3); vi) a purification unit (5); vii) a supercharger (7); viii) means for sending air to the compressor, means for sending compressed air to the purification unit and means for sending at least one portion of the purified air to the supercharger; ix) means for sending air to a column of a system of columns (17, 19); x) means for withdrawing at least one liquid (27) from the system of columns optionally after having stored it in a storage tank; xi) means for sending the liquid to the pump in order to pressurize it; xii) means for sending the pressurized liquid to the first exchanger; xiii) means for withdrawing the vaporized liquid from the first exchanger; xiv) means for sending the pressurized liquid to the second exchanger; and xv) a tank (9) for storing gaseous air under pressure, the tank being connected to the outlet of the air supercharger, characterized in that the storage tank (9) is connected to the outlet of the air supercharger via an expansion valve. 